Custom image size not appearing in srcset value

I have defined a custom image size using Code Snippets plugin like this:

add_image_size( 'hero_image', 1280, 853, true );

It appears in the list of available thumbnail sizes:

Have a field group with a Single Image-type of custom field attached to Posts.

For a test post, set a 1920 x 1282 px image as the hero image.

In the Template for single posts, have

<?php
    $image = rwmb_meta( 'single_image_gkaiaq9nbl6' );

    printf( '<img srcset="%s" src="%s" alt="%s">',
            $image['srcset'],
            $image['full_url'],
            $image['alt'],
    );
?>

On the frontend the HTML output is like this:

<img srcset="https://oxygen.local/wp-content/uploads/2021/12/jay-bird-ornithology-species-fauna-6862629-150x150.jpg 150w,
             https://oxygen.local/wp-content/uploads/2021/12/jay-bird-ornithology-species-fauna-6862629-300x300.jpg 300w,
             https://oxygen.local/wp-content/uploads/2021/12/jay-bird-ornithology-species-fauna-6862629-100x100.jpg 100w"
    src="https://oxygen.local/wp-content/uploads/2021/12/jay-bird-ornithology-species-fauna-6862629.jpg"
    alt="Jay Bird Ornithology Species Fauna  - jggrz / Pixabay">

100x100 thumb is being shown on desktop viewport widths and it is obviously blurry.

I am wondering why my custom image size is not being added to srcset.

Thanks in advance.